VLSM Fundamentals

VLSM Fundamentals

Introduction

Objective

The primary goal of this page is to provide a simple explanation to the techniques for designing and planning LANs using VLSM. The materials presented here are presented for someone who may be at the first course level in the Networking Academy CCNA Exploration curriculum. Knowing how to subnet networks using VLSM quickly is a necessary skill needed as you prepare for your CCNA certifications.

Definitions

VLSM - Variable-length subnet masking. This is a more practical way of subnetting a network by efficiently using the Host bits in the subnet.

CIDR - Classless Inter-Domain Routing. This is based on VLSM subnetting of Class A, B and C networks.

Typical Classful Networks

This is the 'older' method of creating subnetworks. This is not an efficient use of IP addresses within a network. It is actually wasteful of IP addresses because all the subnets have the same number of hosts.

Remember that when you subnet a classful network all of the networks will have the same number of hosts because they have the same subnet mask.

Basic CIDR understanding

There are also a range of IP addresses you can only used privately, not on the internet.

These are the RFC 1918 private IP's:

Class A 10.0.0.0 to 10.255.255.255

Class B 172.16.0.0 to 172.32.255.255

Class C 192.168.0.0 to 192.168.255.255

1 examine the number of bits borrowed for Host addresses.

2 observe the most interesting octet. This will allow you to determine the subnet mask.

3 determine the network increment (size of the network blocks) 256 - subnet mask = block size.

the number of Networks is figured out by 2 to the Nth power - 2 ( N standing for the number of network bits borrowed).

Remember that the number of networks cannot include all 0's or 1's. All 0's is the network address and the broadcast address is all 1's.

Steps to subnet in your head

1

2...

The following

• Postfix
• Send mail

VLSM Design

VLSM is the process where you are subnetting a subnet using varying subnet masks.

Your IP plan will determine the basic layout of the networks. There are multiple solutions possible. It is really up to the network administrator to satisfy the requirements for the environment. It is your choice as to which address ranges you will apply to each subnet.

A good IP plan requires forethought and planning. Evaluate the number of hosts on the current or existing networks and remember to include possible future networks.

IP Subnet Zero.

Steps for creating an IP plan using VLSM

• First examine your networks and determine how many Host bit will be needed to satisfy the largest network

• Next, pick a subnet to use for the next largest network

• Then pick the next largest network to configure, until all networks are accounted for

• Determine network numbers for all serial links

Implementing VLSM

Make sure that your network assignments and subnet masks are accurate and that they fit your topology.

Verify that your IP addresses and subnet masks to not crossover any binary and octal bit boundaries. If they are incorrect, you will receive an error when configuring router interfaces.